BDNF and major depressive disorder: Butyric acid, in particular, acts as a potent histone deacetylase (HDAC) inhibitor in colonocytes and CNS neurons: it increases histone acetylation at the promoter of the BDNF (brain-derived neurotrophic factor) gene, promoting BDNF transcription—BDNF is critical for neuronal survival and synaptic plasticity, and its downregulation is linked to depression (23, 43).